The effective control of flammable gases, especially flammable gases in confined spaces, is important in many contexts. For example, fuel tanks for liquid fuel will develop a fuel vapor mixture in the ullage space of the fuel tank. If the vapor mixture includes a suitable amount of fuel and oxidant (such as molecular oxygen supplied in air), the vapor mixture may be flammable. Large fuel tanks may be found in large vehicles, such as aircraft and ships, and may be found at storage facilities. In addition to liquid fuel storage, transport, and use, flammable gas mixtures may be a consideration in chemical processing, oil refining, mining, power production, heating, metal fabrication, and operations which involve combustible particulate such as sawdust, metal, flour, and grain. In some contexts, one may want to verify that an environment is not flammable and/or whether a gas mixture is flammable
Various sensors may be used to detect potentially flammable gas mixtures. These sensors generally fall into one of four categories: (1) catalytic combustion sensors, (2) infrared absorption sensors, (3) flame ionization sensors, and (4) oxygen sensors. Catalytic combustion sensors include catalysts to encourage combustion reactions. Degradation of the catalysts is significant and causes catalytic combustion sensors to be inaccurate or to need persistent calibration. Infrared absorption sensors monitor specific wavelengths of light associated with optical absorption of known flammable gas components (e.g., a fuel molecule). If the gas composition is complex (having may molecular species) and/or if there are several varying species, the infrared absorption spectra may be difficult to reliably interpret. Flame ionization sensors mix a gas sample with a known amount of flammable gas and ignite the mixture with a pilot flame. Use of flammable gas and a pilot flame limits the applicability of flame ionization sensors to situations in which the hazards of the sensor can be isolated from the gas being tested. Oxygen sensors may incorporate high temperature zirconia sensors that may pose an ignition hazard in flammable mixtures due to the temperature of the zirconia. Other types of oxygen sensors incorporate optical detection via a luminescent probe sensitive to the concentration of oxygen. Luminescent probes may degrade, causing limitations similar to catalytic combustion sensors. Complex mixtures may cause artifacts or otherwise interfere with the luminescent probes. Yet other types of oxygen sensors incorporate electro-galvanic fuel cell sensors that have an electrode that is rapidly consumed by exposure to oxygen.
One method to minimize the risk of a flammable environment in the ullage space of a fuel tank is to flush the ullage space with ‘inert’ gas. This process may be referred to as inerting and may be called inertion. The inert gas is selected to reduce the concentration of oxidant in the ullage space and may not be entirely inert. The inert gas may include oxidant at a low enough concentration that, when mixed with fuel vapor, the mixture is not flammable. Examples of inert gases for fuel tank inertion include nitrogen, nitrogen-enriched air, steam and carbon dioxide. Target oxygen concentrations in the ullage space depend on the fuel constituents (e.g., for jet A fuel, less than 12% (by volume) is considered sufficiently non-flammable). However, in some applications, the oxygen concentration in the ullage space and the flammability of the gas in the ullage space cannot be reliably confirmed because existing flammability sensors are ill-suited. For example, aircraft fuel tanks may hold large quantities of jet fuel and vapor that has a complex assortment of molecular constituents. Additionally, aircraft fuel tanks are subject to a broad range of temperature and pressure conditions. The types, amounts, and concentrations of vaporous fuel species and dissolved gases are affected by temperature and pressure. Therefore, the amount of inert gas needed to achieve the target concentration may vary greatly during a flight.
In conventional aircraft fuel tank inerting systems, the inert gas is substantially continuously supplied to the fuel tank in an effort to maintain the oxygen concentration below the target concentration regardless of the conditions in the fuel tank (e.g., amount of fuel, temperature, pressure). Because the non-flammability of the ullage space cannot be ensured in conventional aircraft fuel tank inerting systems, the aircraft design needs to accommodate the potential for flammable mixtures in the fuel tank (at the likely cost of excess weight, fuel efficiency, and maintenance complexity). Additionally, conventional systems supply gas continuously to the fuel tank, which may lead to increased system capacity needs and increased system wear.